1. Field of the Invention
This invention relates to an improved process and apparatus for measuring total fixed nitrogen species in a gaseous mixture by catalytically converting all of the species to NO and measuring the resulting NO concentration by chemiluminescence. More particularly, the improvements include reducing the pressure of the gaseous mixture followed by preheating in an inert non-catalytic preheater prior to catalytic conversion to NO.
2. Brief Description of the Prior Art
An integral part of combustion research and NO.sub.x emission studies is the measurement of total fixed nitrogen species in gaseous effluents, such as automotive exhausts. Generally, the goal is to measure the concentration of combustion product nitrogen oxides (NO.sub.x) including NO.sub.2 and NO, which are considered to be toxic in the environment. These data can then be correlated with variables in the combustion process, for example to devise ways for reducing the amounts of these species in the exhaust effluents.
Methods are known in the art for the conversion of these nitrogen oxides under thermal and catalytic conditions to form primarily NO, and then measuring the resulting NO concentration by the technique of NO chemiluminescence.
U.S. Pat. No. 3,647,387, for example, describes the chemiluminescent detection of NO produced from pyrolyzing nitrogen compounds by contacting gaseous nitrogen species at 1000.degree.-1250.degree. K. with a copper or platinum catalyst.
Similarly, U.S. Pat. No. 3,919,397 describes the preparation of a gaseous mixture containing NO.sub.2 and NO for chemiluminescent measurement by first passing the mixture through a series of parallel alumina tubes containing a resistivity heated platinum wire to convert all of the NO.sub.2 to NO.
Not only are nitrogen oxides of interest in combustion research, but also is the concentration of NH.sub.3, and HCN, species which are also present in combustion effluents and especially from fuel-rich mixtures.
EPM Report No. 223-1 (March 1976) describes the efficiency of various catalysts in the conversion of various species to NO for chemiluminescent analysis. Platinum is described as being an effective catalyst for converting NH.sub.3 to NO and also described is the enhancement of this conversion at lower pressures and the associated catalytic effect of stainless steel equipment.
It is, however, generally recognized in the field that there is no rapid, accurate and reproducible technique for the measurement of NH.sub.3 or HCN in combustion effluents, especially in rich combustion gases. Further, we have found that currently available commercial instruments, which use stainless steel as the NO.sub.x converter housing are not satisfactory for measuring NH.sub.3 or HCN concentration, particularly when using nitrogen as the diluent gas.